1. Field of the Invention
This invention relates to an optical system adapted for endoscopes and more particularly for endoscopes for infant inspection or industrial use.
2. Description of the Prior Art
It is preferable that the endoscope for infant inspection or industiral use is as fine as possible. Therefore, an endoscope wherein such graded refractive index lens as a Selfoc lens (trade name) is used for the objective lens and relay lens has been developed. For example, as shown in FIG. 1, the optical system of such conventional endoscope is formed of an objective 1, a relay lens 2 consisting of a graded refractive index lens connected to the objective lens 1, a field lens 3 and an eyepiece 4 so that an image O' of an object O may be formed by the objective 1 on its rear end surface and may be relayed through the relay lens 2 to be formed on its rear end surface as an image O" which may be magnified and observed with an eyepiece system consisting of the field lens 3 and eyepiece 4. However, in the objective 1 consisting of the graded refractive index lens, the chromatic aberration can not be corrected and therefore has been so large as to be, for example, 40.mu.(c-line to f-line) in the lateral chromatic aberration and 100.mu. (c-line to f-line) in the longitudinal chromatic aberration. Also, in this objective 1, the monochromatic aberration can not be corrected and therefore there has been a problem that the resolving power is low. Further, in this objective 1, there is today no angle of view more than 60 degrees and therefore there has been a problem that no wide angle of view can be selected. Also, in this objective 1, the part in which the light ray height is low is so short as a characteristic of the graded refractive index lens that it is difficult to bend the optical axis by contriving said part and, as a result, there has been a problem that the oblique viewing is very difficult. Further, there have been problems that, as a characteristic of the graded refractive index lens, in case the entire end surface is utilized to form an image, the light amount on the peripheral side of the visual field will become zero and no photograph will be taken, stripes will appear on the image, the image will be disturbed, the peripheral edge of the visual field will be cracked and the peripheral side of the visual field will be very hard to see. Also, there has been a problem that, in the case of the optical system shown in FIG. 1, if a field stop is provided on the rear surface of the relay lens 2, the focus deviation will be added within the relay lens 2 and therefore it will be difficult to make the field stop and image position coincide with each other. There have been problems that, in such conventional optical system, as no aperture stop has been provided, even an unnecessary light bundle will come in and, as a result, a flare will be produced and the contrast will deteriorate.